Dark matter may be far more complicated than scientists once believed. A new study suggests it could consist of at least two different kinds of particles that slowly separate over time, with heavier particles sinking toward the centers of galaxies and lighter ones drifting outward. This simple idea could explain several puzzling cosmic observations that have frustrated astronomers for years, from unusually diffuse dwarf galaxies to surprisingly dense dark matter clumps that bend light through gravitational lensing.